1. Field of the Invention
This invention relates generally to silicone based adhesive compositions and, more particularly, this invention relates to silicone/organic adhesive copolymers and compositions, and devices made therefrom for controlled release of bioactive or chemical agents.
2. Description of Related Art
Pressure sensitive adhesives and other adhesives containing silicone or organic polymers, including graft copolymers, are well known. However, prior art adhesives tend to be either hydrophobic or hydrophilic in nature, rendering them unsuitable for applications wherein conditions incompatible with the adhesive's hydrophobic or hydrophilic nature are encountered.
Also, many prior art adhesives based on silicone or organic graft copolymers are of types which are incapable of being formulated to satisfy particular requirements with respect to adhesion to specific substrates, liquid and/or gas permeability or impermeability, and release of active agents to the substrate at a controlled rate, or removal of undesirable agents from the substrate.
There has long been a need for adhesives which are capable of adhesion to a wide variety of substrates and which are adaptable to various environments and conditions. Of special interest are pressure sensitive adhesives which offer numerous advantages in industrial as well as health-related applications.
Adhesives are widely used in agricultural, industrial or health-related applications wherein it is desired to release an active agent such as a herbicide, insecticide, drugs or other bioactive agents at a controlled rate for sustained time periods. Such adhesives ideally should be capable of being loaded with sufficiently high concentrations of the agent to be released for the desired application, should release the agent into the surrounding atmosphere or underlying substrate in a controlled fashion, preferably at a uniform rate, over the desired period of use and, especially in agricultural or health-related fields, not directly harm or contribute to harm of the underlying substrate. The adhesion of such materials should be sufficiently great that the adhesive remains adhered to the substrate, yet not be so great as to cause trauma to the substrate upon removal of the adhesive.
In industrial applications, pressure sensitive adhesives are widely used in composite structures and other applications to bond two or more layers or surfaces of similar or dissimilar materials to each other. Such adhesives should provide as high adhesion to each substrate as possible in order to ensure the integrity of the structure over the desired time period of use and under the temperature and moisture conditions encountered in use.
The use of pressure sensitive adhesives in transdermal drug delivery devices is well known. Generally, in such devices a layer of a pressure sensitive adhesive loaded with a drug or other bioactive agent is adhered to a backing layer, e.g. a bandage, which is applied to a user with the pressure sensitive adhesive directly in contact with the user's skin.
Such devices are generally packaged with a release liner disposed over the surface of the pressure sensitive adhesive layer.
Ideally, the drug or other agent loaded into the pressure sensitive adhesive layer will be released therefrom and migrate through the user's skin at a desired rate in order to provide a uniform rate of drug delivery to the user. A uniform rate of drug delivery is desirable in that, ideally, the concentration of a drug in a user's bloodstream should be maintained above the minimum therapeutic level and below the level at which undesirable side effects occur.
Other desirable characteristics of pressure sensitive adhesives useful in transdermal drug delivery devices are sufficiently high adhesiveness to skin, to the backing layer and to the release liner to ensure the integrity of the drug delivery device. However, the adhesion between the adhesive layer and the release liner must not be so high as to hinder removal of the release liner, and the adhesion to skin must not be so high as to cause trauma during removal of the device.
Also, the device as a whole including the backing layer and adhesive layer should be sufficiently permeable to the passage of oxygen and water vapor to avoid damage to the underlying skin. The adhesive should not lose its adhesive properties on contact with water, nor should the adhesive lose its adhesive properties upon aging.
The rate of release of a bioactive agent should be uniform with respect to time. The permeability coefficient of the bioactive agent through the skin varies with the individual user depending on race, the position on the body, and other factors known in the art. The permeability coefficient is the product of the diffusion coefficient and the solubility coefficient associated with the materials of construction.
Another advantageous property of an adhesive useful in a transdermal drug delivery device is a sufficiently high tack, which is a measure of how quickly adhesion between the adhesive layer and substrate occurs.
Ideally, the pressure sensitive adhesive should not only act merely as a carrier for the bioactive agent but should also enhance the rate of release of the agent.
Some prior art transdermal drug delivery devices utilize adhesives which exhibit excessively high or poorly controlled adhesion to the skin, so as to cause trauma such as skin tearing, removal of epidermis, pulled hair, or bruises upon removal.
A major problem with prior transdermal drug delivery devices is their inability to provide either highly uniform release rates as the concentration of the drug or other bioactive agent in the device decreases, or a release rate which is sufficiently high to provide a therapeutic concentration of drug or other bioactive agent in the user's blood without attaining a concentration which is so high as to cause undesirable side effects.
Others have insufficient permeability to oxygen and water vapor such that edema, blistering, or wrinkling or discoloration of skin occurs.
Further, another major problem with currently available medical grade pressure sensitive adhesives is their tendency to dry out and lose all or most of their tack upon aging. This is especially evident in pressure sensitive adhesives loaded with drugs containing amine functionality.